Polycrystalline silicon photoelectric transducer and process for its production

Abstract: This invention intends to provide an inexpensive polycrystalline silicon solar cell having a large grain size polycrystalline semiconductor layer grown on a low-cost metal-level silicon substrate, and a process for its production. The polycrystalline silicon solar cell comprises a metal-level silicon substrate 101, a metal oxide layer 102 formed thereon and a polycrystalline silicon layer 105 formed on the metal oxide layer. The process for producing the polycrystalline silicon solar cell comprises the steps of i) depositing a metal oxide layer 102 on a metal-level silicon substrate 101, ii) depositing a silicon layer 103 on the surface of the metal oxide layer, iii) depositing a cap layer 104 on the surface of the silicon layer and melting the silicon layer by heating from the upper part of the cap layer, followed by solidification to form a polycrystalline silicon layer, and iv) removing the cap layer and forming a semiconductor junction on the surface of the polycrystalline silicon layer.

Description: BACKGROUND OF THE INVENTION Field of the Invention This invention relates to a polycrystalline silicon photoelectric conversion device and a process for its production. More particularly it relates to a polycrystalline silicon photoelectric conversion device having a good energy conversion efficiency, and a process for its production. Related Background Art As drive energy for various equipments and as power sources systematically connected with commercial power, solar cells serving as photoelectric conversion device are widely researched. Solar cells make use of pn junctions in their functional parts, and Si is commonly used as semiconductors constituting the pn junctions. From the viewpoint of efficiency for converting light energy into electromotive force, it is preferred to use monocrystalline silicon. From the viewpoint of achievement of larger area and lower cost, amorphous silicon types are considered advantageous. In recent years, use of polycrystalline silicon is studied for the purpose of achieving a low cost comparable to amorphous silicon types and a high energy conversion efficiency comparable to monocrystalline silicon types. In processes hitherto proposed, however, bulk polycrystals are sliced into sheet members when used, and hence it is difficult to make their thickness not larger than 0.3 mm. Thus, they have a thickness excessively large enough to absorb light in a sufficient amount, resulting in no satisfactory effective utilization of materials in this regard. In other words, in order to make cost reduction, they must be made to have much smaller thickness. Accordingly, it is attempted to form thin films of polycrystalline silicon by using thin-film forming techniques such as chemical vapor deposition (CVD), according to which, however, crystal grain size is equal to film thickness and can only be several-hundredths to scores µm (microns). Thus, under existing circumstances, such films have a low energy conversion efficiency also when compared with the products produced by slicing bulk polycrystalline silicon. Under such circumstances, it is also attempted to irradiate polycrystalline silicon thin films with laser light to melt them to effect recrystallization so that the crystal grain size can be enlarged. This, however, can achieve no satisfactory cost reduction and makes it difficult to carry out stable manufacture. Meanwhile, it is proposed to form crystalline silicon films on low-cost substrates by zone melting recrystallization (ZMR) in a thickness large enough to absorb sunlight (a Japanese language publication, Hamamoto et al., "Crystal Defects and Solar Cell Characteristics in Polycrystalline Si Thin Films", the third workshop on High-efficiency Solar Cells in Toyama, 1992, p.20). Fig. 5 cross-sectionally illustrates a solar cell fabricated by the above process. On a metallurgical grade silicon substrate 502, which is an inexpensive substrate with a low purity, an insulating layer (SiO 2 ) 503 as an impurity barrier, a poly-Si (polycrystalline silicon) film 507 and an SiO 2 cap layer are formed in this order, which is subjected to zone melting recrystallization (ZMR) to make the poly-Si film larger in grain size. After the cap layer has been removed, silicon is epitaxially grown on this poly-Si layer by atmospheric pressure CVD. Thus, a polycrystalline silicon thin film with a layer thickness of about 50 µm and a grain size of several mm to several cm is formed. On this thin film, a pn junction 504 is formed by diffusion and an antireflection film 505 and a surface electrode 506 are further formed to produce a polycrystalline silicon thin film cell on the surface side. Thereafter, the silicon substrate and also the SiO 2 insulating layer are selectively removed by etching applied from the back to bare the back surface of the polycrystalline silicon thin film, and a back electrode 501 is formed thereon. Thus, a polycrystalline silicon solar cell is produced through the above steps. However, the polycrystalline silicon solar cell fabricated by this process have a problem on energy conversion efficiency which should be more improved to a higher conversion efficiency. Moreover, according to this process, the polycrystalline silicon layer and the substrate is electrically insulated because of the presence of the SiO 2 layer between them, and hence it is necessary to carry out etchback from the back of the substrate until the SiO 2 layer is laid bare, where the SiO 2 layer is further removed to bare the back surface of the polycrystalline silicon layer so that the electrode material can be deposited thereon to make electrical connection. Thus, this process has another problem of complicated processing steps. SUMMARY OF THE INVENTION An object of the present invention is to solve the above problems involved in the prior art to provide a polycrystalline silicon photoelectric conversion device formed of crystals with a large grain size and a good quality, and process for its production. The present invention provides a photoelectric conversion device comprising a metallurgical grade silicon substrate (hereinafter often "MG-Si"), a layer of conductive metal oxide formed on said MG-silicon substrate, and a polycrystalline silicon layer (hereinafter often "poly-Si") formed on said layer of metal oxide. The present invention also provides a process for producing a polycrystalline silicon photoelectric conversion device, comprising the steps of; forming a layer of conductive metal oxide on a MG-silicon substrate; forming a silicon film on said layer of metal oxide; and crystallizing the silicon film to form a polycrystalline silicon layer. The invention thus constituted makes it possible to provide a polycrystalline silicon photoelectric conversion device that has good characteristics and can be fabricated with ease and to provide a process for producing such a device. BRIEF DESCRIPTION OF THE DRAWINGS Figs. 1A to 1D schematically illustrate a main process for producing a thin-film polycrystalline silicon solar cell according to the present invention. Figs. 2A to 2E schematically illustrate a process for producing a thin-film polycrystalline silicon solar cell according to the present invention. Figs. 3A and 3C schematically illustrate lamp irradiation devices used in the process of the present invention, and Figs. 3B and 3D are graphs to show examples of temperature sequence in heating. Figs. 4A to 4E schematically illustrate a process for producing a heterojunction type solar cell. Fig. 5 is a cross section of a conventional thin-film polycrystalline silicon cell. DESCRIPTION OF THE PREFERRED EMBODIMENTS How the present invention operates will be described with reference to Figs. 1A to 1D. Main technical features of the present invention are as follows: As shown in Figs. 1A to 1D, (a) on a MG-Si substrate 101 a metal oxide 102 is deposited by a means such as sputtering (Fig. 1A), and thereafter, (b) a silicon layer 103 is formed by CVD or the like and a cap layer 104 comprising NSG (non-doped silica glass) or the like is further deposited thereon by CVD or the like (Fig. 1B). Then, (c) the silicon layer is heated by lamp irradiation from the upper part and melted so as to effect recrystallization so that its grain size is enlarged (Fig. 1C), and also (d) optionally after removal of the cap layer a recrystallized silicon layer 105 thus formed is subjected to liquid-phase epitaxy to increase its thickness (Fig. 1D). The present inventor has discovered that a polycrystalline silicon layer with a large grain size and a good film quality can be formed when a silicon layer deposited on a metal oxide such as ZnO is recrystallized by lamp heating, and thus has accomplished the present invention. More specifically, it is presumed that in the step (c) of recrystallization there is not so much difference in the coefficient of thermal expansion between the Si layer, the metal oxide layer and the MG-Si substrate, and hence the stress applied to the recrystallized Si layer and the MG-Si substrate little comes into question, so that good poly-Si and good electrical properties can be obtained. In addition, in the step (a), the use of the metal oxide (a metal oxide having a conductivity, such as ZnO) enables electrical connection between the recrystallized Si layer and the underlying MG-Si substrate, so that any time-consuming, troublesome steps such as the etchback of substrates that is required when SiO 2 film is used become unnecessary. The present invention will be described below in detail. - Substrate 101 - As the substrate 101 used in the present invention, a MG-silicon which can achieve electrical connection, has substantially the same coefficient of thermal expansion as that of high-purity silicon which is used as semiconductor and has a low purity, specifically, contains 1 ppm to 2% of impurity elements is inexpensive and can be used relatively with ease. - Metal Oxide Layer 102 - The metal oxide used in the present invention may be selected from those having a melting point higher than that of silicon and having a conductivity. For example, ZnO, NiO and V 2 O 3 are used. In particular, ZnO is preferred in view of its readiness to handle, a small difference in coefficient of thermal expansion with respect to Si and its less tendency to produce stress. Thickness of the metal oxide 102 depends on the materials of metal oxides, melting or recrystallizing conditions and so forth. Stated approximately, it may preferably be in the range of from 0.5 to 10 µm. The metal oxide layer may also preferably have a specific resistance of 10 -3 to 1 Ω·cm taking account of conductivity. - Si 103 to be recrystallized - The silicon layer deposited on the metal oxide layer 102 may be non-monocrystalline, i.e., amorphous or crystalline, or may be a mixture of amorphous and crystalline. The silicon layer may be deposited by any process including atmospheric pressure CVD, LPCVD, plasma-assisted CVD such as high-frequency plasma-assisted CVD or microwave plasma-assisted CVD, photo-CVD, vapor deposition and sputtering. The silicon layer may preferably have a thickness approximately in the range of from 0.5 to 50 µm. The silicon layer having been recrystallized may preferably have a layer thickness of several tens microns or less and a grain size of several hundreds µm to several mm. When recrystallized, maximum temperature may preferably be set at 1,410 to 1,450 C, and retention time on that occasion may preferably be from 5 seconds to 120 seconds. When the silicon layer having been melted is caused to solidify, the rate of temperature drop is most important and may preferably be from 0.1 to 5°C per second. - Cap Layer 104 - The cap layer 104 on the silicon layer 103 used in the process of the present invention is formed in order to prevent the deposited silicon layer from melting to round or evaporate. As layers that may cause no deterioration of film quality of the deposited silicon layer, NSG, PSG (Phosphorus Silicate Glass), BSG (Boron Silicate Glass), Si 3 N 4 or ZnO, or any combination thereof, may be used. Such layers can be formed using an atmospheric pressure CVD reactor, a LPCVD reactor or a sputtering reactor. In particular, the use of PSG or BSG makes it possible also to dope the silicon layer with impurities in the course of melting to determine its conductivity type. Thickness of the cap layer, which depends on materials and combination thereof, may preferably be controlled in the range of from 1 to 5 µm. - Crystal Growth Process - The process for crystal growth used in the present invention may include LPCVD (low-pressure CVD), LPE (Liquid-Phase Epitaxy), atmospheric pressure CVD, plasma-assisted CVD, photo-CVD and sputtering. In view of growth rate and crystallinity, it is preferred to use liquid-phase epitaxy. The range of growth temperature in the liquid-phase epitaxy, which depends on the type of solvents, may preferably be controlled to be from 850°C to 1,050°C when Si and Sn are used as a solute and a solvent, respectively. The degree of super-cooling may preferably be several °C or so, and the rate of temperature drop may preferably be controlled in the range of from 0.1 to 5°C per second. Impurities are doped in the surface of the resulting poly-Si for the purpose of forming a semiconductor junction thereon. The impurities are doped by ion implantation or thermal diffusion, and are selected from P, As, Sb and so forth as n-type impurities and B, Al and so forth as p-type ones. Alternatively, a semiconductor layer having a conductivity type different from that of the poly-Si may be deposited on the surface of the poly-Si. The depth of such junction or thickness of the semiconductor layer, which depends on the quantity of impurities doped, may suitably be in the range of from 0.01 to 1 µm, and more preferably from 0.02 to 0.5 µm. In view of requirements for characteristics of solar cells, it is suitable for the recrystallized Si layer or poly-Si layer formed by a crystal growth process to have a final layer thickness of from 10 to 200 µm, and more preferably from 20 to 200 µm. Experiment 1 Melting and recrystallizing silicon layers on metal oxides: On the surface of a 0.5 mm thick MG-Si substrate 201, ZnO was deposited by conventional sputtering in a thickness of 2 µm as a metal oxide layer 202 (Fig. 2A). On the layer thus formed, a silicon layer 203 was deposited in a thickness of 6 µm using a conventional vacuum deposition reactor. The Si layer thus formed was examined by X-ray diffraction to reveal that it was amorphous silicon. On such a Si layer formed on the MG-Si substrate, an NSG film 204 was deposited as a cap layer in a thickness of 2 µm using an atmospheric pressure CVD reactor (Fig. 2B). Next, the Si layer was melted by light irradiation using a halogen lamp to effect recrystallization (Fig. 2C). A schematic view of a lamp irradiation device and the temperature sequence for melting and recrystallizing the si layer as used in this step are shown in Figs. 3A and 3B, respectively. In Fig. 3A, reference numeral 301 denotes a substrate; 302, a quartz chamber; 303, a lamp; 304, cooling water; and 305, a reflector. After the light irradiation was completed, the NSG film on a recrystallized Si layer 203' was removed with hydrofluoric acid, and the states of the surface and cross section of the recrystallized Si layer was observed using an optical microscope and a scanning electron microscope to confirm that a relatively smooth Si layer was obtained and its layer thickness was substantially the same as that before melting. Crystal grain boundaries were acturized by Secco etching to reveal that the grain size of the recrystallized Si layer 203' had been enlarged by a size of several mm in maximum and also the etch pits on the surface of the Si layer were in a density of about 10 5 /cm 2 . Experiment 2 In the same manner as in Experiment 1, NiO was deposited on the MG-Si substrate 201 by conventional sputtering in a thickness of 2 µm to form the metal oxide layer 202, and the Si layer 203 was deposited thereon in a thickness of 8 µm using a conventional vacuum deposition reactor and then melted by lamp heating to carry out recrystallization. In this instance also, a similar recrystallized Si layer was obtained. Experiment 3 On the recrystallized Si layer obtained in Experiment 1, crystal growth was further made by liquid-phase epitaxy. The crystal growth was made using a conventional slide type liquid-phase epitaxy assembly and using Sn and Si as a solvent and a solute, respectively, at a growth initiation temperature of 950°C, a super-cooling degree of 3°C and a temperature drop rate of 0.5°C/min and for a growth time of 60 minutes (Fig. 2D). After the epitaxy was completed, the surface and cross section of the growth layer were observed in the same manner as in Experiment 1, using an optical microscope and a scanning electron microscope to confirm that a relatively smooth Si layer 205 was obtained and its layer thickness was about 50 µm. Its grain size also took over the size of the underlying recrystallized Si layer and also the etch pits on the surface of the Si layer thus grown were in a density of about 5 x 10 4 pits /cm 2 . Experiment 4 Into the surface portion of the large grain size Si crystal thin film on the MG-Si substrate obtained in Experiment 3, phosphorus (P) was implanted by ion implantation under conditions of 80 keV and 1 x 10 15 ions/cm 2 , followed by annealing at 800°C for 30 minutes to form an n + -layer 206. Then, on that layer, collector electrodes 207 (Cr: 0.02 µm/Ag: 1 µm/Cr: 0.004 µm) and a transparent electrode 208 (ITO: 0.085 µm) were formed by vacuum deposition (Fig. 2E). In regard to solar cells produced in this way, having the structure of large grain size Si crystal thin film - ZnO - MG-Si, the I-V characteristics under light irradiation of AM 1.5 (100 mW/cm 2 ) were measured. As a result, in cell area of 2 cm 2 , the open-circuit voltage was 0.55 V, the short-circuit photocurrent was 32 mA/cm 2 and the fill factor was 0.75, where a conversion efficiency of 13.2% was obtained. Thus, it was proved that a large grain size Si thin film can be formed by superposingly forming the metal oxide layer and the Si layer on the MG-Si substrate and melting the Si layer by lamp irradiation to effect recrystallization, and thus a solar cell (photovoltaic device, Device-1) having good characteristics can be fabricated. Experiment 5 For comparison, a recrystallized Si layer was formed in the same manner as in Experiment 1 except that the metal oxide layer 202 was replaced with a layer formed of SiO 2 . The surface and cross section of the recrystallized Si layer were observed to reveal that a relatively flat Si layer was obtained like that in Experiment 1 but etch pits formed by Secco etching were in a density of as large as 10 7 pits/cm 2 . Experiment 6 A solar cell (Device-2) was fabricated using what was obtained in Experiment 5, under the same conditions as in Experiment 4. To compare Device-1 obtained in Experiment 4 with Device-2 obtained in Experiment 6 and set up electrical connection, the MG-Si, SiO 2 and ZnO of Device-1 and Device-2 at right beneath the device were partly peeled off, and thereafter metal electrodes were formed to compare conversion efficiency. It was found that Device-1 attained conversion efficiency about twice that of Device-2 and the film quality of recrystallized Si layers greatly influenced the characteristics of solar cells. Fabrication of the desired solar cells by carrying out the process of the present invention will be described below in greater detail by giving Examples. The present invention is by no means limited by these Examples. Example 1 A large grain size polycrystalline Si solar cell was produced on a MG-Si substrate in the same manner as in Experiment 4. On the surface of a 0.5 mm thick MG-Si substrate, ZnO was deposited by conventional sputtering to form a metal oxide layer in a thickness of 2 µm. On this layer, an n-type Si layer was deposited in a thickness of 20 µm using a conventional vacuum deposition reactor. The Si layer thus formed was examined by X-ray diffraction to reveal that it was amorphous silicon. On the Si layer formed on such a MG-Si substrate, an NSG film was deposited as a cap layer in a thickness of 2 µm using an atmospheric pressure CVD reactor, followed by light irradiation using a halogen lamp (25 kW) as a heat source for recrystallization, according to the temperature sequence as shown in Fig. 3B, so as to melt the Si layer to effect recrystallization. After the light irradiation was completed, the NSG film on a recrystallized Si layer was removed with an aqueous solution of hydrofluoric acid, and then boron (B) was implanted into the surface portion of the Si crystal thin film by ion implantation under conditions of 20 keV and 1 x 10 15 ions/cm 2 , followed by annealing at 800°C for 30 minutes to form a p + -layer. Then, on that layer, collector electrodes (Ti/Pd/Ag: 0.04 µm/0.02 µm/1 µm) and an ITO transparent electrode 0.085 µm) were formed on the p + -layer. In regard to solar cells produced in this way, having the structure of large grain size Si crystal thin film - ZnO - MG-Si, the I-V characteristics under light irradiation of AM 1.5 (100 mW/cm 2 ) were measured. As a result, in cell area of 2 cm 2 , the open-circuit voltage was 0.56 V, the short-circuit photocurrent was 25 mA/cm 2 and the fill factor was 0.74, where a conversion efficiency of 10.4% was obtained. Example 2 A polycrystalline Si solar cell having an n + p junction was produced in the same manner as in Example 1. Figs. 2A to 2E show its fabrication process. As the metal oxide layer 202, ZnO was deposited by conventional sputtering in a thickness of 2 µm on the surface of a 0.5 mm thick MG-Si substrate 201 (Fig. 2A). On the layer thus formed, a silicon layer 203 was deposited in a thickness of 10 µm using a conventional LPCVD reactor. The Si layer thus formed was examined by X-ray diffraction to reveal that it was polycrystalline Si having a grain size of about 8 nm. On such a Si layer formed on the MG-Si substrate, a BSG film was deposited as a cap layer 204 and also as a layer determining the conductivity type of the Si layer, in a thickness of 2 µm using an atmospheric pressure CVD reactor (Fig. 2B). Next, using the lamp irradiation device as shown in Fig. 3A, the Si layer was melted by light irradiation of halogen lamps (25 kW) according to the temperature sequence as shown in Fig. 3B, to effect recrystallization (Fig. 2C). Thereafter, the NSG film was removed with an about 5% aqueous solution of hydrofluoric acid, and then crystal growth was made using a conventional liquid-phase epitaxy assembly of a slide type boat system and using Sn as a solvent, under conditions as shown below, to obtain a large grain size Si crystal thin film 205 (Fig. 2C). The above epitaxy was carried out in an atmosphere of hydrogen at a growth initiation temperature of 950°C, a super-cooling degree of 3°C and a temperature drop rate of 0.5°C/min and for a growth time of 55 minutes (Fig. 2D). The Si crystal thin film and recrystallized Si layer thus obtained had a final layer thickness of abut 50 µm. Next, phosphorus (P) was thermally diffused into the surface of the Si crystal layer at a temperature of 900°C using POCl 3 as a diffusion source to form an n + -layer 206 and obtain a junction in a depth of about 0.5 µm. The dead layer produced by the thermal diffusion on the n + -layer thus formed was removed by etching to obtain a junction in a depth of about 0.2 µm having a surface density of phosphorus. On the n + -layer, collector electrodes 207 and an ITO transparent electrode 208 were formed in the same manner as in Example 1 (Fig. 2E). In regard to solar cells produced in this way, having the structure of n + -Si - p-Si crystal thin film - ZnO - MG-Si, the I-V characteristics under light irradiation of AM 1.5 (100 mW/cm 2 ) were measured. As a result, in cell area of 2 cm 2 , the open-circuit voltage was 0.56 V, the short-circuit photocurrent was 34 mA/cm 2 and the fill factor was 0.73, where a conversion efficiency of 13.9% was obtained. Example 3 A p + -microcrystalline silicon/polycrystalline silicon heterojunction type solar cell was produced in the same manner as in Examples 1 and 2. Figs. 4A to 4E show a fabrication process of the heterojunction type solar cell. First, ZnO was deposited by reactive sputtering in a thickness of 2 µm on the surface of a MG-Si substrate 401 (Fig. 4A). On that layer, a silicon layer 403 was further deposited in a thickness of 12 µm by LPCVD, and also thereon a PSG film 404 was deposited in place of the BSG film 204 in Example 2, as a cap layer 404 and also as a layer determining the conductivity type of the Si layer, in a thickness of 2 µm using an atmospheric pressure CVD reactor (Fig. 4B). Then, the Si layer was melted by light irradiation of halogen lamps (25 kW) according to the temperature sequence as shown in Fig. 3B, to effect recrystallization (Fig. 4C). Here, after 100 seconds shown in Fig. 3B, the temperature may be dropped in the same way or, in view of improvement in production efficiency, the film may be quenched. After the NSG film was removed with an aqueous solution of hydrofluoric acid, and then crystal growth was made using a conventional liquid-phase epitaxy assembly of a slide type boat system and using Sn as a solvent and Si as a solute (0.48% by weight) in an atmosphere of hydrogen at a growth initiation temperature of 950°C, a super-cooling degree of 3°C and a temperature drop rate of 0.5°C/min and for a growth time of 55 minutes (Fig. 4D). The Si crystal thin film and recrystallized Si layer thus obtained had a final layer thickness of abut 50 µm. Subsequently, in place of the n + -layer 206 in Example 2, a p-type microcrystalline Si layer 406 was formed on the Si crystal layer 405. The p-type microcrystalline Si layer was deposited on the Si crystal surface in a thickness of 0.02 µm using a conventional plasma-assisted CVD reactor under conditions as shown in Table 1 below. The microcrystalline Si film thus formed had a dark conductivity of about 10 S cm -1 . Gas flow rate ratio Substrate temperature Pressure Discharge power SiH 4 /H 2: 1 cc/20 cc 250°C 66.66 Pa (0.5 Torr) 20 W B 2 H 6 /SiH 4 :2 x 10 -3 As a transparent conductive film 408, ITO was formed by electron beam deposition in a thickness of about 0.1 µm. On this film, collector electrodes 407 (Cr: 0.02 µm/Ag: 1 µm/Cr: 0.004 µm) were formed by vacuum deposition (Fig. 4E). In regard to p + -microcrystalline silicon/ polycrystalline silicon heterojunction type solar cells produced in this way, the I-V characteristics under light irradiation of AM 1.5 were measured. As a result, in cell area of 1.6 cm 2 , the open-circuit voltage was 0.60 V the short-circuit photocurrent was 35.2 mA/cm 2 and the fill factor was 0.68, where a conversion efficiency of as high as 14.4% was obtained. Example 4 A large grain size Si crystal solar cell was produced in the same manner as in Example 1 according to the process as shown in Fig. 2. In the manner as previously described, ZnO was deposited on the surface of a MG-Si substrate by sputtering in a thickness of 3 µm. On the ZnO layer thus formed, an n-type Si layer was deposited in a thickness of 40 µm using a conventional atmospheric pressure CVD reactor. Next, an NSG film was deposited as a cap layer in a thickness of 2 µm using an atmospheric pressure CVD reactor, followed by light irradiation using a halogen lamp according to the temperature sequence as shown in Fig. 3B, so as to melt the Si layer to effect recrystallization. After the light irradiation was completed, the NSG film on a recrystallized Si layer was removed with an aqueous solution of hydrofluoric acid. In order to form a p + -layer on the surface of the Si crystal thin film, BSG was deposited using an atmospheric pressure CVD reactor, followed by RTA (Rapid Thermal Annealing). The BSG deposited was in a layer thickness of about 0.6 µm and the RTA was carried out under conditions of 1,050°C for 60 seconds. The junction thus formed was in a depth of about 0.2 µm. After the BSG was removed with an aqueous solution of hydrofluoric acid, the surface of the Si crystal layer was thinly oxidized (about 0.01 µm) by dry oxidation, and the oxide film formed was etched by photolithography. On that surface, collector electrodes (Ti/Pd/Ag: 0.04 µm/0.02 µm/1 µm) and an ITO transparent electrode (0.085 µm) were formed on the p + -layer by electron beam deposition using a metal mask. In regard to crystal thin-film solar cells produced in this way, the I-V characteristics under light irradiation of AM 1.5 were measured. As a result, in cell area of 2.5 cm 2 , the open-circuit voltage was 0.58 V, the short-circuit photocurrent was 35 mA/cm 2 and the fill factor was 0.72, where a conversion efficiency of 14.6% was obtained. Example 5 A large grain size Si crystal solar cell was produced in the same manner as in Example 1 according to the process as shown in Fig. 2. In the manner as previously described, NiO was deposited on the surface of a MG-Si substrate by sputtering in a thickness of 3 µm. On the surface of the NiO layer thus formed, an n-type Si layer was deposited in a thickness of 40 µm using an atmospheric pressure CVD reactor. Next, a ZnO film was deposited as a cap layer in a thickness of 2 µm using a sputtering reactor, followed by light irradiation of halogen lamps using the device as shown in Fig. 3C, according to the temperature sequence as shown in Fig. 3D, so as to melt the Si layer to effect recrystallization. In the device shown in Fig. 3C, heat sources are provided unsymmetrically in the upper and lower directions (light intensity on the upper side is greater than light intensity on the lower side). Hence, the Si layer can be efficiently recrystallized while preventing the MG-Si substrate from melting. After the light irradiation was completed, the ZnO film on a recrystallized Si layer was removed with an aqueous solution of hydrochloric acid. In order to form a p + -layer on the surface of the Si crystal thin film, BSG was deposited using an atmospheric pressure CVD reactor, followed by RTA (Rapid Thermal Annealing). The BSG deposited was in a layer thickness of about 0.6 µm and the RTA was carried out under conditions of 1,050°C for 60 seconds. The junction thus formed was in a depth of about 0.2 µm. After the BSG was removed with an aqueous solution of hydrofluoric acid, the surface of the Si crystal layer was thinly oxidized (about 0.01 µm) by dry oxidation, and the oxide film formed was etched by photolithography. On that surface, collector electrodes (Ti/Pd/Ag: 0.04 µm/0.02 µm/1 µm) and an ITO transparent electrode (0.085 µm) were formed on the p + -layer by electron beam deposition using a metal mask. Characteristics of crystal thin-film solar cells produced in this way were measured. As a result, the same characteristics as in Example 4 were obtained. As described above, according to the present invention, it has been proved that large grain size Si thin films can be formed when the Si layer deposited on the metal oxide layer on the MG-Si substrate is melted by lamp irradiation to effect recrystallization, and inexpensive solar cells with a mass productivity can be produced by the use of such thin films. According to the present invention, it has also become possible to fabricate crystal thin-film solar cells with good characteristics, on MG-Si substrates through a simple process. This makes it possible to commercially provide inexpensive, good-quaility thin-film type solar cells with a mass productivity. This invention intends to provide an inexpensive polycrystalline silicon solar cell having a large grain size polycrystalline semiconductor layer grown on a low-cost MG-silicon substrate, and a process for its production. The polycrystalline silicon solar cell comprises a MG-silicon substrate 101, a metal oxide layer 102 formed thereon and a polycrystalline silicon layer 105 formed on the metal oxide layer. The process for producing the polycrystalline silicon solar cell comprises the steps of i) depositing a metal oxide layer 102 on a MG-silicon substrate 101, ii) depositing a silicon layer 103 on the surface of the metal oxide layer, iii) depositing a cap iayer 104 on the surface of the silicon layer and melting the silicon layer by heating from the upper part of the cap layer, followed by solidification to form a polycrystalline silicon layer, and iv) removing the cap layer and forming a semiconductor junction on the surface of the polycrystalline silicon layer.

Claims: A photoelectric conversion device comprising a metallurgical grade silicon substrate, a layer of metal oxide having electrical conductivity on said metallurgical grade silicon substrate, and a polycrystalline silicon layer on said layer of metal oxide. The photoelectric conversion device according to claim 1, wherein said metallurgical grade silicon substrate has an impurity density in the range of from 1 ppm to 2%. The photoelectric conversion device according to claim 1, wherein said metal oxide having electrical conductivity is selected from at least one of ZnO, NiO and V 2 O 3 . The photoelectric conversion device according to claim 1, wherein said metal oxide has a specific resistance of from 10 -3 to 1 Ω·cm. The photoelectric conversion device according to claim 1, wherein said layer of metal oxide has a thickness of from 0.5 to 10 µm. The photoelectric conversion device according to claim 1, wherein said polycrystalline silicon layer has a thickness of from 10 to 200 µm. The photoelectric conversion device according to claim 1, wherein said polycrystalline silicon layer is provided with a semiconductor junction. The photoelectric conversion device according to claim 7, wherein said semiconductor junction is a pn junction. The photoelectric conversion device according to claim 1, wherein said polycrystalline silicon layer has a grain size of from about several hundred µm to several mm. The photoelectric conversion device according to claim 1, wherein said polycrystalline silicon layer is provided thereon with a non-monocrystalline semiconductor layer. The photoelectric conversion device according to claim 10, wherein said non-monocrystalline semiconductor is a microcrystalline semiconductor. A process for producing a photoelectric conversion device, comprising the steps of: forming a layer of metal oxide having electrical conductivity on a metallurgical grade silicon substrate ; forming a silicon film on said layer of metal oxide; and crystallizing the silicon film to form a polycrystalline silicon layer. The process for producing a photoelectric conversion device according to claim 12, wherein the crystallization is carried out using a heat energy. The process for producing a photoelectric conversion device according to claim 13, wherein said heat energy is supplied by lamp heating. The process for producing a photoelectric conversion device according to claim 13, wherein said heat energy is supplied from the silicon film side. The process for producing a photoelectric conversion device according to claim 13, wherein said heat energy comprises a first part supplied from the silicon film side and a second part that is smaller than said first part from the silicon film side and is supplied from the metallurgical grade silicon side. The process for producing a photoelectric conversion device according to claim 12, wherein a semiconductor junction is formed on said polycrystalline silicon layer. The process for producing a photoelectric conversion device according to claim 12, wherein a nonmonocrystalline semiconductor layer is formed on said polycrystalline silicon layer. The process for producing a photoelectric conversion device according to claim 18, wherein said non-monocrystalline semiconductor layer is microcrystalline. The process for producing a photoelectric conversion device according to claim 17, wherein said semiconductor junction is formed by doping said polycrystalline silicon layer with impurities. The process for producing a photoelectric conversion device according to claim 17, wherein said semiconductor junction is formed by providing on said polycrystalline silicon layer a semiconductor layer containing impurities. The process for producing a photoelectric conversion device according to claim 21, wherein said semiconductor layer is non-monocrystalline. The process for producing a photoelectric conversion device according to claim 22, wherein said non-monocrystalline semiconductor layer is microcrystalline. The process for producing a photoelectric conversion device according to claim 12, which further comprises the steps of; forming a cap layer on said silicon film; and removing the cap layer after the step of crystallization. The process for producing a photoelectric conversion device according to claim 24, wherein said polycrystalline silicon layer is made to have a larger thickness by a crystal growth process carried out after removing said cap layer. The process for producing a photoelectric conversion device according to claim 25, wherein said crystal growth process is liquid-phase epitaxy. The process for producing a photoelectric conversion device according to claim 26, wherein said liquid-phase epitaxy employs Sn as a solvent and Si as a solute. The process for producing a photoelectric conversion device according to claim 24, wherein said cap layer is formed of a material selected from at least one of non-doped silicate glass, phosphosilicate glas, borosilicate glas, Si 3 N 4 and ZnO. The process for producing a photoelectric conversion device according to claim 24, wherein said cap layer has a thickness of from 1 to 5 µm. The process for producing a photoelectric conversion device according to claim 13, wherein the heat energy in the step of crystallization is controlled at a temperature drop rate of from 0.1 to 5°C/sec. The process for producing a photoelectric conversion device according to claim 26, wherein said liquid-phase epitaxy is carried out at a growth temperature of from 850°C to 1,050°C. The process for producing a photoelectric conversion device according to claim 12, wherein said metallurgical grade silicon has an impurity density in the range of from 1 ppm to 2%. The process for producing a photoelectric conversion device according to claim 12, wherein said metal oxide having electrical conductivity is selected from at least one of ZnO, NiO and V 2 O 3 . The process for producing a photoelectric conversion device according to claim 12, wherein said metal oxide has a specific resistance of from 10 -3 to 1 Ω·cm. The process for producing a photoelectric conversion device according to claim 12, wherein said metal oxide layer has a thickness of from 0.5 to 10 µm. The process for producing a photoelectric conversion device according to claim 12, wherein said polycrystalline silicon layer has a thickness of from 10 to 200 µm. The process for producing a photoelectric conversion device according to claim 17, wherein said semiconductor junction is a pn junction. The process for producing a photoelectric conversion device according to claim 18, wherein a pn junction is formed of said polycrystalline silicon layer and said non-monocrystalline layer formed on said polycrystalline silicon layer. The process for producing a photoelectric conversion device according to claim 12, wherein said polycrystalline silicon layer has a grain size of from about several hundred µm to several mm.

Inventors: NISHIDA SHOJI, NISHIDA, SHOJI, NISHIDA, SHOJI, C/O CANON KABUSHIKI KAISHA
Applicants: CANON KK, CANON KABUSHIKI KAISHA
Application year: 1994
Main classification: H01L31
IPC codes: H01L31, H01L21